This invention relates to a contact for a vacuum interrupter and, more particularly, to a contact for a vacuum interrupter having both improved anti-welding characteristic and improved voltage withstanding characteristic and a process for producing the same.
Contacts for a vacuum interrupter for carrying out large current interruption or rated current make and break in a high vacuum utilizing an arc diffusion property in a vacuum, are constituted of two opposing contacts, i.e., stationary and movable contacts. Principal characteristics required for such a contact for a vacuum interrupter are anti-welding property, voltage withstanding capability, and current interrupting property. Important requirements other than these fundamental requirements are low and stable temperature rise and low and stable contact resistance. However, these requirements contradict each other and therefore it is impossible to meet all of the requirements by a single metal. Accordingly, in many contacts which have been practically used, at least two elements which compensate mutually inadequate performance thereof have been used in combination to develop contact which are suitable for specific uses at a large current, at a high voltage or at other conditions. Contacts having excellent characteristics have been developed. However, demands for a contact for a vacuum interrupter which withstands higher voltage and larger current have increased, and a contact for the vacuum interrupter which entirely meets such requirements has not been obtained.
For example, Japanese Patent Publication No. 12131/1966 discloses a Cu-Bi alloy containing no more than 5% of an anti-welding component such as Bi. This reference describes that the Cu-Bi alloy can be used as a contact which is used at a large current. However, the solubility of Bi in the Cu matrix is extremely low, and therefore segregation occurs. Further, the surface roughening after current interruption is large and it is difficult to carry out processing or forming.
Japanese Patent Publication No. 23751/1969 discloses the use of a Cu-Te alloy as a contact which is used at a large current. While this alloy alleviates the problems associated with the Cu-Bi alloy, it is more sensitive to an atmosphere as compared with the Cu-Bi alloy. Accordingly, the Cu-Te alloy lacks the stability of contact resistance or the like.
Furthermore, although both the contacts formed from the Cu-Te alloy and those from the Cu-Bi alloy have excellent anti-welding properties in common and can be used sufficiently in prior art moderate voltage fields in respect to voltage withstanding capability, it has turned out that they are not necessarily satisfactory in applying to higher voltage fields.
On the other hand, a known contact-forming material for a vacuum interrupter is a Cu-Cr alloy containing Cr. The Cu-Cr alloy contact exhibits preferred thermal characteristic of Cr and Cu at a high temperature and therefore it has excellent characteristics in respects of high voltage withstanding capability and large current stability. That is, the Cu-Cr alloy is widely used as a contact wherein high voltage withstanding characteristic is compatible with large capacity interruption.
However, the Cu-Cr alloy exhibits greatly inferior anti-welding characteristic as compared with the Cu-Bi alloy containing no more than about 5% of Bi which has been generally widely used as the contact for the interrupter.
The welding phenomenon occurs by any of the following two causes: (a) the contact melts by Joule heat generated at the contacting surfaces of the contacts and thereafter solidifies and; (b) the contact gasifies by arc discharge generated in the instant of make and break, and thereafter solidifies.
In any cases, the Cu-Cr alloy forms fine grains of Cr and Cu having no more than 1 micrometer when it is solidified. Thus, the Cu-Cr alloy forms a layer having a thickness of the order of from several micrometers to several hundreds of micrometers in such a state that Cr fine grains and Cu fine grains are intermingled. Generally, super-refinement of a structure is one of factors which contribute to improvement of the strength of the material. In the Cu-Cr alloy, this is true. When the strength of the superfine Cu-Cr layer is larger than the strength of the matrix of the Cu-Cr alloy and the matrix strength exceeds designed opening force, welding generates.
Accordingly, operation mechanism by which a vacuum interrupter formed by using a contact of a Cu-Cr alloy is driven requires a larger opening force as compared with the vacuum interrupter formed by using the Cu-Bi alloy contact, and therefore the vacuum interrupter formed by using the Cu-Cr alloy contact is disadvantageous in respects of miniaturization and economy.
Japanese Patent Publication No. 41091/1986 discloses a contact of a Cu-Cr-Bi alloy wherein Bi is added to a Cu-Cr alloy in order to improve the anti-welding property of the Cu-Cr alloy. While this Cu-Cr-Bi alloy contact generally improves the anti-welding property of the Cu-Cr alloy to a certain extent, the addition of Bi remarkably embrittles the stuck, reduces voltage withstanding characteristic and increases restrike generation probability.
As described above, the contact of the Cu-Cr-Bi alloy has generally improved anti-welding property as compared with the contact of the Cu-Cr alloy. However, problems remain in respects of voltage withstanding characteristic and restrike generation.